Almost all soil slopes on land are initially unsaturated in nature. In order to determine the factor of safety of unsaturated slopes accurately, it is essential to calculate transient seepage and hence pore water pressures in the soil. To do so, one has to measure the so-called soil-water characteristic curve (SWCC), which defines the relationship between the soil suction and either the water content or the degree of saturation of the soil. Conventionally SWCC of a soil is measured by means of a pressure plate extractor in which no confining stress can be applied and volume change of the soil specimen is assumed to be zero, although it is recognized that the stress state of a soil will affect SWCC theoretically and volume change of the soil specimen will take place when there is a change of soil suction and stress. The Applicant developed and published a one-dimensional volumetric pressure plate extractor in which the total net normal stress can be controlled one-dimensionally and the axial deformation of the soil specimen is measured. The measured stress-dependent SWCC is now called SDSWCC [Ng, C. W. W. and Pang, Y. W. (2000). Influence of stress state on soil-water characteristics and slope stability. Journal of Geotechnical and Geoenvironmental Engineering, ASCE. Vol. 126, No. 2. 157-166.].
Since the fundamental concepts behind the Applicant's previous design of the one-dimensional stress controllable pressure plate extractor and the “Fredlund cell” (a product from GCTS Company, model SWC-150, Fredlund SWCC Device) are identical, both of these two devices are suffered from not being able to control stress-state in three-dimensional manner to simulate actual field conditions more closely and correctly. An axis-translation technique is employed in both of these two devices. Hence, the suction is matric suction instead of total suction. Secondly, volume changes of soil specimen cannot be measured accurately because a gap can form between the soil specimen and an oedometer ring at high suctions which cannot be accounted for by measuring vertical displacement of the soil specimen only. Thirdly, the range of suctions that can be applied is limited by the air-entry value of the ceramic disk used (i.e., typically less than 500 kPa). Therefore, any measured SWCC may not be very relevant to some actual field stress conditions and is not accurate. Also since any measured SWCC is limited to a suction range of less than 500 kPa, this will reduce the applicability of the SWCC in engineering problems and soil types severely.
To improve the accuracy of volume change measurement, a new “total volume change measuring system” is developed and published by the Applicant and his colleagues [Ng, C. W. W., Zhan, L. T. and Cui, Y. J. (2002). A new simple system for measuring volume changes in unsaturated soils. Canadian Geotechnical Journal. Vol. 39, No. 3, 757-764]. In the 2002 design of total volume measuring system, the volume change of the soil specimen during extraction is measured by measurement of difference of water level in a reference tube and in the inner cell, in particular at the neck of the inner cell.
Based on current widely accepted theoretical framework, unsaturated soil behavior and properties are recognized to be governed by at least two stress-state variables, i.e. net normal stress and metric suction. At present, no simple and accurate experimental system is available to measure soil-water characteristic curves under various three-dimensional stress states and high total suctions, up to 8,000 kPa.
This significance of this new invention is to overcome the above shortcomings by controlling stress state of a soil specimen three-dimensionally, by measuring the actual total volume changes of the soil specimen accurately, and by making use of relative humidity (RH) to control total suction up to 8,000 kPa.